This invention relates to a print control method and a printer and, more particularly, to a print control method and a printer according to an inkjet printing method which prints images on a printing medium by using a printhead for performing print by using inks.
As inkjet printers, monochrome inkjet printers for performing monochrome printing by using an ink of one color and color inkjet printers for performing color printing by using inks of a plurality of colors are commercially available. Color inkjet printers generally print color images by using inks of three colors, i.e., yellow (Y), magenta (M), and cyan (C). Some color inkjet printers also use black (K) in addition to these three colors.
Recently, with the spread of personal computers, information processors, and communication apparatuses, printers which perform digital image printing by using an inkjet printhead have rapidly found widespread use as one image forming (printing) device of these apparatuses. Also, as the image quality and the ability to process color images of these information and communication apparatuses have been improved, demands on the image quality and the ability to process color images of printers as output devices of these apparatuses are increasing. To increase the printing speed, printers of this sort use a printhead in which orifices and ink channels are integrated at a high density, as a printhead (to be referred to as a multihead hereinafter) having an integrated array of a plurality of print elements. A color printhead generally has dedicated head units for inks of cyan, magenta, yellow, and black. However, the density of integration of ink orifices and ink channels cannot be unlimitedly increased. As a consequence, an ink dot shape (graininess) becomes conspicuous in a highlight portion of an image. This is a problem in high-quality printing of images.
As an approach to realize high-quality image printing by improving the construction of an apparatus, a so-called multi-drop printing method has been proposed. In this method, instead of increasing the integration density of orifices and ink channels, the dot diameter of a discharged ink is decreased, and these small dots are printed a plurality of times in one pixel in accordance with the print density. In this multi-drop method, the image quality of a highlight portion is slightly improved because the dot diameter is smaller than usual. However, the size of ink droplets to be discharged cannot be unlimitedly decreased due to the relationship with the stability of discharge. This limits an improvement of the image quality.
As another approach by which the image quality is improved without increasing the integration density of orifices and ink channels, a printing method has been proposed in which thin inks of the same color with different dye concentrations are used such that a highlight portion of an image is printed with a thin ink to make the ink dot shape inconspicuous. Printers of this sort can perform printing by using either regular thick inks or thin inks by exchanging ink tanks (or cartridges) storing the thick inks for ink tanks (or cartridges) storing the thin inks.
Unfortunately, when printing is performed by exchanging ink tanks storing inks of the same color with a plurality of different densities in an inkjet printer capable of printing by exchanging these ink tanks, or when printing is performed by exchanging ink cartridges using inks of the same color with a plurality of different densities, printing is not always performed under the same conditions, and this results in the following problems.
(1) Ink Mixing
When ink tanks are exchanged, if an ink used after the exchange has a density different from that of an ink used before the exchange, the ink before the exchange remaining inside a printhead (e.g., a common ink compartment or an ink supply passage) sometimes has an influence on the ink density after the exchange. For example, when an ink tank containing a thick ink is exchanged for an ink tank containing a thin ink, the thick ink remaining inside a printhead mixes in the thin ink to increase the density to be higher than a normal thin ink density. If printing is performed in this state, the density is initially high and gradually decreases as the printing progresses. Finally, the normal density of the thin ink is obtained. This is not a problem at all when inks having the same density are used. That is, this is an intrinsic phenomenon when inks having different densities are used by exchanging ink tanks.
Also, when ink cartridges are exchanged, an ink used before the exchange adheres to a wiping member or a capping member and sometimes has an influence on the ink density after the exchange. For example, an ink cartridge of a thick ink is exchanged for an ink cartridge of a thin ink, the thick ink adhering to the above member mixes with the thin ink. This has an influence on an image printed after a recovery operation such as wiping or capping. This is also an intrinsic phenomenon when printing is performed by the same printer by using inks having different densities.
(2) Density Variation
The optical reflection density of a thin ink changes largely with a change in dye concentration when compared to a regular thick ink. FIG. 2 shows a change of the reflection density as a function of a change in dye concentration. Referring to FIG. 2, the reflection density is normalized by assuming that the dye concentration of the regular thick ink is 1.0 and the corresponding reflection density is 1.0. When the dye concentration of a thick ink changes between 0.8 and 1.0, the reflection density changes between 0.93 and 1.0; i.e., the change width of the reflection density is 0.07. In contrast, when a thin ink prepared by diluting the regular thick ink having a dye concentration of 1.0 five times is used, a change in reflection density with a change in dye concentration from 0.1 to 0.3 centered on a dye concentration of 0.2 is 0.30 (=0.59-0.29). The reflection density of a thin ink is more sensitive, than that of a thick ink, to the same change width of the dye concentration; i.e., the reflection density of the thin ink changes larger than that of the thick ink. Accordingly, when the dye concentration of an ink in an ink discharge nozzle not used in printing in a printhead changes because, e.g., the nozzle is dried, the reflection density of the thin ink varies larger than that of the thick ink.
The foregoing is a problem when ink tanks or ink cartridges containing inks of the same color with different densities are interchangeably used. This problem is unavoidable when high-quality color images are formed by using inks of the same color with a plurality of different densities.